An unexplained force is pulling on distant spacecraft. Researchers have come to this conclusion after a thorough analysis of the deep-space probes' trajectories.

We've been working on this problem for several years, and we have accounted for everything we could think of

Dr John Anderson, Nasa

It could be just a tiny unnoticed effect in the spacecraft themselves, but scientists warn it could also be the first hint that modifications need to be made to our understanding of the force of gravity.

"It is almost as if the probes are not behaving according to the known law of gravity," said Dr John Anderson, of the American space agency's (Nasa) Jet Propulsion Laboratory, and lead scientist on the study.

He said: "We've been working on this problem for several years, and we have accounted for everything we could think of."

Great detail

The unexplained force appears to be acting on four deep-space probes scattered around the Solar System.

Pioneer 10 was launched towards the outer planets in 1972. It is now well beyond Jupiter but still in radio contact with Earth.

By studying the Doppler shift (the "stretching") of the radio signals from the probe, scientists have been able to calculate how fast the craft is travelling. Since 1980, its trajectory has been mapped in very great detail.

The puzzle is that Pioneer 10 is slowing more quickly than it should.

It was initially suggested that this might be due to the force from a tiny gas leak
or that it was being pulled off course by the gravity of an unseen Solar System object.

Unseen body

The mystery deepened further when an analysis of the trajectory being followed by its sister spacecraft, Pioneer 11, launched in 1973, showed that it too was being subjected to the same mysterious effect.

But Pioneer 11 is on the opposite side of the Solar System from Pioneer 10, about 22 billion km (about 14 billion miles) away. This means the effect cannot be the gravitational effect of some unseen body.

Add to all this hints that the same unexplained effect might have been acting on the Galileo spacecraft on its journey to Jupiter, and the Ulysses spaceprobe that is circling the Sun, and you have a Solar System-wide puzzle.

In a report soon to be published in a major astronomical journal, Dr Anderson and colleagues have carried out an impressive study of the state of the Pioneer spacecraft and all the tiny forces to which they could be subjected.

Planets unaffected

"Our analysis strongly suggests that it is difficult to understand how any of these mechanisms can explain the magnitude of the observed behaviour of the Pioneer anomaly," the team says.

It has been suggested that the spacecraft tracking data have shown a deviation in the force of gravity that is apparent only across vast distances. It has also been pointed out that the strength of the effect seems to be related to two of the Universe's physical constants: the speed of light and the speed of the expansion of the Universe.

But others have dismissed this as being too fanciful, arguing that if the Pioneer anomaly was really indicative of a change in our understanding of gravity, then it would be apparent in the orbits of the planets around the Sun - which it is not.

The effect is as yet unexplained and with all four affected probes never to return to Earth for analysis, it may well remain that way.